Paleoecology of an early eocene mammalian fauna from paleosols in the clarks fork basin, Northwestern Wyoming (U.S.A.)

Abstract

Winkler, D. A., 1983. Paleoecology of an early Eocene mammalian fauna from paleosols in the Clarks Fork Basin, northwestern Wyoming (U.S.A.). Palaeogeogr., Palaeoclimmatol., Palaeoecol.,.The paleoecology and depositional history of an early Eocene vertebrate assemblage dominated by small mammals is analyzed at a richly fossiliferous locality in the Willwood Formation, northwestern Wyoming. Systematic surface sampling using a quadrat net, screen washing, and quarrying of fresh sediment show that all vertebrate material in these fluvial deposits is contained in dark gray mudstones representing the A horizons of paleosols. Seven such paleosols occur in 22 m of stratigraphic section. The fauna includes 34 species of mammals, and also birds, reptiles, amphibians, fishes and gastropods. All vertebrate fossil material is fragmented, but it shows no evidence of transportation or current sorting. Surface weathering and possibly carnivore activity are responsible for the fragmentation and for an overabundance of dense and compact skeletal elements.Detailed surface collections were made to obtain multiple samples from the different bone beds (paleosols). Principle component analysis shows that differences in the type and abundance of fossils from the various collecting areas are related to differences between contained bone beds, not differences along single bone beds. One paleosol has a red B horizon and six have an orange B horizon. The gray-over-red colored paleosol is faunally distinct from gray-over-orange paleosols. It contains the only specimens of two large, archaic mammals, and no very small mammalian taxa.Cluster analysis on the occurrence of vertebrate taxa is used to define subsets of the fauna that may represent animals living in different habitats. Separate clusters are formed by a group of the most common terrestrial herbivorous mammals and the crocodilian Allognathosuchus, and by a group of small, possibly arboreal herbivores and carnivores.The relative abundance of each mammalian species was initially calculated from a sample representing a minimum of 300 individual animals collected from the surface. The surface sample is dominated by mammals that are medium-sized for the fauna. Screen washing yields a greater relative abundance of very small mammals than was found on the surface. This indicates that the surface sample is biased against small forms. Corrected relative abundances based on both surface and washing samples show that the fauna is dominated numerically by very small mammals (one multituberculate species and several insectivores). Screen washing of fresh matrix is the best means of sampling a locality for paleoecological purposes. 1Present address:

Taphonomy of a late Cretaceous mammal-bearing microvertebrate assemblage from the Deccan inter-trappean beds of Naskal, peninsular India

“…All the skeletal elements, including those of fish, amphibians, turtles, lizards, snakes, crocodiles, dinosaurs, and mammals, are disarticulated, fragmentary, bear pre-burial fractures on some of the larger bones, exhibit flaking at higher magnification, and are characterized by a spongy structure. These features strongly support the interpretation that the microvertebrate assemblage experienced subaerial exposure before final burial (Bown and Kraus, 1981;Winkler, 1983;Bown and Beard, 1990;Gunnell, 1994). Coprolites, though rare, are well preserved and if transported would have shown signs of abrasion or disaggregation.…”

“…In the last 15 years, good qualitative works, stemming from the previous studies, have appeared and ascribed the origin of microvertebrate remains to avian (Mayhew, 1977;Dodson and Wexlar, 1979;Kusmer, 1990) or mammalian (Mellett, 1974;Andrews and Evans, 1983;Maas, 1985) predators or as a consequence of sorting and concentration by hydraulic forces (Dodson, 1973;Wolff, 1973;Korth, 1979;Wood et al, 1988: Eberth, 1990. Irrespective of their nature, most of these studies are restricted to Neogene and Recent mammalian faunas ( Wolff', 1973;Behrensmeyer, 1978Behrensmeyer, , 1982Korth, 1979: Denys, 1985: Badgley, 1986Denys et al, 1987: Pratt, 1989: Kusmer, 1990) and very few workers have focused attention on older deposits (Dodson, 1971Winkler, 1983;Maas, 1985;Sander, 1987: Wood et al. 1988Eberth, 1990).…”

Spatial niche partitioning in dinosaurs from the latest cretaceous (Maastrichtian) of North America

“…Additionally, reworked specimens typically lose their skeletal association and reworked specimens are often redeposited as isolated elements or part of a larger bonebed, both of which were excluded from this analysis. In addition, fossils found in floodplain deposits are generally interpreted as representing the site of death because the energy needed to remove elements from the channel and deposit them on the floodplain rarely exists [18,49,50]. Finally, numerous actualistic taphonomic studies on vertebrate skeletal hard parts indicate that out-of-life-habitat transport generally affects relatively few individuals in a given fossil assemblage ( [47] and references therein).…”

Distribution and stratigraphic correlation of upper Paleocene and lower Eocene fossil mammal and plant localities of the Fort Union, Willwood, and Tatman formations, southern Bighorn Basin, Wyoming

“…Although screen washing at some sites may significantly affect the composition by revealing an unsuspected abundance of microfaunal taxa (for example, Winkler, 1983), it is unlikely that this procedure would fundamentally alter the diversity indices in table 15. Still, the assemblages display quite a large range in species richness, and one that is generally proportional to sample size.…”